1. Field of the Invention
The present invention pertains to telephone systems and more particularly to an arrangement for use in electronic telephone systems to precondition the telephone line to eliminate transients which might otherwise affect an electronic central office switching system or PBX.
2. Description of the Prior Art
Telephone systems have traditionally employed low frequency high voltage signals, usually superimplosed upon a DC bias, to activate electromechanical tone ringers, transformers, etc. for subscriber signalling. Interruption of such circuits normally occurs in the periodic application of ringing voltage and the transfer of the line circuit from the control and supervisory mode, to transmission status and vice versa. During such switching functions, reactive loads are interrupted and transient voltage and current spikes are generated.
Recently designed telephone switching systems make extensive use of electronic circuitry which is less tolerant of spurious transient signals than previous electro-mechanical systems. In order to protect such electronic switching systems against these false and unpredictable conditions, the use of transient suppression devices has been extensive. Such devices are required in several locations within the switching apparatus and on a per line basis. The requirement for transient suppression devices is expensive in terms of both cost and space. More frequently they affect the transmission quality or signal response requiring additional remedial action.
A number of electronic circuits have been developed for applying or removing ringing current from the telephone line. Such circuitry is disclosed in U.S. Pat. Nos. 3,187,106, 3,192,323, 3,378,643 and 3,668,331. None of the noted patents however deals with the conditioning and establishment of desirable line electrical states in order to prevent the generation of harmful transients, their purpose being to provide such features as interpretation and regulation of line supervisory signals and improving the efficiency of ring generator utilization.
Obviously a circuit that will prevent generation of transients is a far more desirable corrective measure, than to protect the equipment against such occurrences. Accordingly, it is the object of the present invention to describe an arrangement for both sensing the opportune circuit conditions and the switching of the circuit to accomplish inter and intra circuit rearrangement without inducing spurious electrical transient signals.